Nanomaterials have high surface area and high mechanical strength. Owing to their exceptional chemical/physical stability, nanomaterials have been widely used in harsh/corrosive and elevated temperature environments.
On the other hand, activated carbon, polymer, copolymer, oligomer and/or organic molecules can be used as heavy metal sorbents via electrostatic, π-π and/or σ-π associations, namely physical associations.
Most metal sulfides, such as mercury sulfide, are highly stable inorganic salts with an extremely low stability constant (HgS: log K=−22.3). Such mercury sulfide (HgS) typically has very low solubility in water, and isn't toxic unlike most of the other mercury compounds and can be stored and transported without any problem.
In a large number of combustion processes, mercury is transferred to the gaseous phase and must be removed by subsequent treatment. At present in industrial fields, two methods are primarily used for waste-gas purification: 1) The gas is treated with sulfuric acid (90%) at 200° C. The mercury (I) sulfate formed is deposited in wash towers. Further mercury removal is performed by after purification with sodium sulfide. 2) The cooled and dusted roast gas is treated with a mercury(II) chloride solution, and mercury precipitates as mercury(I) chloride (Hg2Cl2). Mercury is removed from the wash liquid by treatment with sodium sulfide. Part of the mercury(I) chloride produced is oxidized to mercury(II) chloride with gaseous chlorine and returned to the process. The final level of mercury in the waste gas is 0.05-0.1 mg/m3.
At current coal-fired power plants, injections of powdered active carbon (PAC) or active carbon impregnated with sulfur, fluoride, chloride, bromide, iodine and other compounds for mercury removing are predominantly applied as mercury adsorbents (U.S. Pat. No. 5,372,619 and U.S. Pat. No. 6,953,494); however, the halogens are mostly physically absorbed on/in the surface or pore of active carbon, which could be de-absorbed during the storage and shipment, especially at an elevated temperature. Furthermore, the halogen elements usually are highly corrosive and volatile. When halogen-impregnated active carbons are injected into flue gas prior to electrostatic separation precipitator, the resulting mercury sorbent is mixed with great volume of fly ash, which needs to be disposed of in a landfill. Accordingly the methods of PAC Hg removal technologies are not exactly “mercury removal”, the target mercury is just partitioned and transferred to the huge amount of solid waste.
Many materials and devices can be used in purification methods to remove mercury and other heavy metals from wastewater. Examples are activated carbon filters, ion exchangers, and electrolysis systems. Mercury and other heavy metals can also be removed from solution by addition of suitable reagents.
To purify scrubber waste water from refuse combined heating and power stations, the additive TMT-15 solution developed by Degussa (U.S. Pat. No. 5,075,444) is used in some cases on an industrial scale. The active substance of the additive consists of trimercapto-s-trazine solution in the form of a sodium salt. Mercury is bound as (C3N3S3Hgx)n, which is stable up to 210° C. and is only sparingly soluble in the elution test. The mercury content is reduced from ca. 4 mg/L to <0.05 mg/L (Degussa Brochure 1982). In order to achieve the best performance, the pH-value of the treated waste water should be adjusted to a neutral or slightly alkaline range, because the complex (C3N3S3Hgx)n of TMT with mercury has certain solubility in acid environments.
For ion exchangers, a distinction can be made between reusable and disposable ones (Rosenzweig, Chem. Eng. (london), 1975, 82, 60; Bergk et al, Chem. 1977, 17, 85). In some cases, sulfur-containing substances are used as active agent, which remove mercury from the solution by forming a mercury sulfide compound.
Small amounts of oil dispersed in the aqueous solution can enhance the content of enriching mercury in the oil phase. The mercury content of the purified aqueous solution is in some cases <10 μg/L. Fine purification is carried out by using a conventional oil-water emulsion splitting unit.